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How To Add CSS Code In MU Plugin

A must-use plugin (also called a ‘mu-plugin’) is a plugin that will always be activated by default, without you needing to do it yourself. To activate a mu-plugin, you just have to upload it in the right directory, and WordPress will automatically know that this plugin must be used. Moreover, a mu-plugin cannot be deactivated: it will always be executed, unless it is uninstalled.

Originally, mu-plugins were only available for WPMU, for those sites which used multiple (multisite) blogs. Back then, a mu-plugin was just a plugin activated for all blogs.

Fun fact, that the WordPress is loading mu-plugins in alphabetical order, before normal plugins.

All mu-plugins must be placed at wp-content/mu-plugins by default, in case /mu-plugins/ folder is not there, feel free to create it.